Skeletal muscle, heart, and brain are high-energy requiring tissues that are severely affected by mitochondrial dysfunction. Recently a novel form of genetic disease affecting mitochondria has been associated with mutations in a mitochondrial protein, CHCHD10 (D10), whose function is still largely unknown. Mutant D10 causes severe autosomal dominant mitochondrial diseases, with diverse phenotypic features, ranging from myopathy to motor neuron disease and frontotemporal dementia. We previously showed that in mitochondria D10 forms a dimeric complex with its paralog protein, CHCHD2 (D2). Interestingly, mutations in D2 are also associated with familial neurodegenerative diseases. To study the manifestations and disease mechanisms of mutant D10 in vivo, we have generated a knock in mouse harboring the first pathogenic D10 mutation reported in humans (S59L, corresponding to mouse S55L). In D10S55L mouse muscle and heart mitochondria, D10 and D2 accumulate and aggregate, leading to mitochondrial dysfunction and degeneration. These abnormalities result in a profound integrated mitochondrial stress response (ISRmt), altering transcriptional profiles and metabolism, and ultimately resulting in fatal cardiomyopathy. Conversely, D10 knock out mice do not manifest mtISR and are phenotypically normal, suggesting that D10S55L causes disease through a toxic mechanism and not a loss of function. In this application, we will study the normal function of D10 and D2 and the mechanisms underlying mitochondrial alterations in D10S55L mice. Since D10 mutations cause neurodegeneration in humans, we will also investigate the involvement of the nervous system in D10S55L mice. We will then identify metabolic and molecular biomarkers to help monitor disease course. Lastly, we will test the effects of pharmacological modulation of ISRmt in D10S55L mice as a therapeutic strategy. The impact of this project will be to facilitate rationale approaches to target disease pathogenesis in patients with D10 mutations, which could be extended to other mitochondrial diseases mediated by ISRmt